With the recent rapid spread of portable electronic devices such as mobile phones, smartphones, electronic books, and portable game machines, secondary batteries for drive power supply have been increasingly required to be smaller and to have higher capacity. Nonaqueous secondary batteries typified by lithium-ion secondary batteries, which have advantages such as high energy density and high capacity, have been widely used as secondary batteries used for portable electronic devices.
A lithium-ion secondary battery, which is one of nonaqueous storage batteries and widely used due to its high energy density, includes a positive electrode including an active material such as lithium cobalt oxide (LiCoO2) or lithium iron phosphate (LiFePO4), a negative electrode formed of a carbon material such as graphite capable of reception and release of lithium ions, a nonaqueous electrolytic solution in which a supporting electrolyte formed of a lithium salt such as LiBF4 or LiPF6 is dissolved in an organic solvent such as ethylene carbonate or diethyl carbonate, and the like. A lithium-ion secondary battery is charged and discharged in such a way that lithium ions in the secondary battery move between the positive electrode and the negative electrode through the nonaqueous electrolyte solution and inserted into or extracted from the active materials of the positive electrode and the negative electrode.
A binder is mixed into the positive electrode or the negative electrode in order that active materials can be bound or an active material and a current collector can be bound. Since the binder is generally an organic high molecular compound such as polyvinylidene fluoride (PVDF) which has an insulating property, the electron conductivity of the binder is extremely low. Therefore, as the ratio of the mixed binder to the active material is increased, the amount of the active material in the electrode is relatively decreased, resulting in the lower discharge capacity of the secondary battery.
Hence, by mixture of a conductive additive such as acetylene black (AB) or a graphite particle, the electron conductivity between active materials or between an active material and a current collector can be improved. Thus, an active material with high electron conductivity can be provided (see Patent Document 1).
Graphene oxide (abbreviation: GO) is mixed with an active material, a binder, a solvent, or the like to make a mixture, and then the mixture is heated, whereby an active material layer can be formed. The graphene oxide in the active material is reduced by the heat treatment, so that the graphene can be an active material that functions as a conductive additive. It has been found that in such an active material layer, a network for electron conduction is formed and excellent electron conductivity is obtained (see Patent Document 2).